1. Technical Field
The present invention relates to subsea lines comprising a number of fluid/gas conducting steel tubes and possibly other elongated elements like electrical conductors and cables,--possibly enclosed within a common outer cover. The invention relates in particular to corrosion protection of subsea lines having steel tubes for conducting control and service fluids for subsea installations, preferably static installations.
2. Description of the Prior Art
With a few exceptions, commercially available stainless steels are not sufficiently corrosion resistant when exposed to sea water. Additional corrosion protection, like cathodic protection will therefore be required also for stainless steel constructions that will--or might--be exposed to sea water.
Existing prior art include very expensive stainless steel umbilicals and zinc-plated steel tubes. The problem has been to achieve longtime corrosion protection of carbon steel umbilicals and tubes. Some prior art solutions are not good enough pricewise. Other solutions do not provide longtime protection. Most prior art solutions include an outer extruded polymer sheath to obtain a socalled `dry` design.
Such arrangement will under certain circumstances be subjected to corrosion attacks, in particular when the tubes are covered with an outer plastic sheath. Mechanical damage to the plastic sheath may, if there is insufficient contact between the tubes and the sheath, lead to water penetration. In such situations, sacrificial anodes on the outside will not give sufficient cathodic protection to the narrow interstices under the plastic sheath.
A common type of umbilical consists of a large center tube with a number of smaller tubes stranded around the large tube. There are, however, also other configurations.
From our UK Patent GB 2 255 104 B there are known subsea lines and umbilicals having corrosion protection satisfying most offshore requirements. The cathodic protection of the stainless steel tubes is obtained by a `built in` sacrificial anode system. The outer surfaces of the small tubes achieve cathodic protection from integrated zinc ires, while the lower surfaces of the tubes are protected by galvanized steel tape. This is, however, also a `dry` design, relying on a nonpenetrable outer cover.